I. Field of the Invention
The present invention relates to an emission computerized tomograph imaging apparatus, and more particularly to a ring type single-photon emission CT imaging apparatus which detects radiation, e.g., gamma ray photons of radioisotope that has been injected into an object under examination such as a patient, so as to process detection signals, thereby obtaining a CT image from a three-dimensional distribution of radioactivity of radioisotope within the object.
II. Description of Prior Art
In general, emission computerized tomography commonly referred to as "ECT" is known from, e.g., U.S. Pat. No. 4,289,965 to Koga et al. issued on Sept. 15, 1981 and U.S. Pat. No. 4,389,569 to Hattori et al. issued on June 21, 1983. ECT is a technique for obtaining an image of the distribution (i.e., spatial position) of radioactivity of radioisotope in a desirable slice normal to the longitudinal axis of a body of a patient within a region of interest thereof by administering to the patient a pharmaceutical compound labelled with the radioisotope, for detecting from outside of the body the gamma radiation emitted by the isotope that has been accumulated in the above region of interest, and for processing the detection signals by a computer to obtain the tomographic image.
In such a known ECT, a plurality of radiation detectors are circumferentially arranged about the longitudinal axis of a body, and directed to the body at different angles within 360 degrees in a given slice of a body so as to detect the radioactivity of radioisotope. Generally, a collimator made of the radioactivity protection material, e.g., lead is positioned in front of the respective detectors so as to selectively receive gamma rays having a predetermined travel direction and a given energy level.
In this system, the sensitivity and resolution of the detector system are influenced to a greater extent by the employed collimators. That is, since the energy level of gamma rays that have commonly been employed in the field of nuclear medicine is rather high, say from tens to hundreds KeV (Kilo-electron volts), a thickness of the collimator's wall must be thick, resulting in the insensitivity and lowered resolution of the detector system.
In the above-described ECT having a plurality of detectors which are positioned in a ring shape around the patient (referred to as "ring type ECT"), especially a single-photon mode ECT, it is required to employ collimators that can selectively receive the gamma rays in the specific directions only. This type of the collimator is known as a parallel hole collimator and a fin shape collimator. As previously described, the employment of such collimators causes resolution and sensitivity to deteriorate. Moreover, such a ring type single-photon ECT is not available to perform the dynamic scanning by which movements of the radioisotope within a body under examination can be measured.
It is therefore an object of the present invention to provide a ring type single-photon ECT imaging apparatus in which electronic collimators, instead of mechanical collimators, are employed to realize the dynamic scanning.
A further object of the present invention is to provide a compact ECT imaging apparatus for counting gamma rays with higher accuracy.